This invention relates generally to semiconductor integrated circuit test apparatus, and more particularly the invention relates to a method and apparatus for measuring operating temperature of the active area of a semiconductor device.
The operating temperature of semiconductor devices, particularly gallium arsenide FETs employed in microwave circuits, is an important parameter in military and space applications. The operating temperature of the active area directly relates to the operating lifetime of the device.
However, semiconductor die are too small to use conventional contact probing techniques to measure their temperature. Three alternative techniques are presently used and are the basis for several commercially available products.
In one technique, a high-magnification microscope is used in conjunction with an infrared detector. The intensity of the infrared radiation from the focused area is a strong function of temperature and can, in principle, be detected and measured. However, different materials have widely varying emissivities which makes the accuracy of any measurement critically dependent on an in situ calibration procedure and on the precise positioning of the sample area. Accuracy and repeatability are difficult to achieve in practical use; however, high-volume manufacturing and screening applications have used the technique because of the nonintrusive nature thereof.
The optical properties of liquid crystals can be employed in monitoring temperature. However, this is essentially a destructive technique which can only be used in sample tests. The test device has to be coated with the crystals and cannot be reused in its normal application. The technique does have the advantage of very high resolution and is typically used to locate hot spots. Only one temperature can be measured per crystal type.
The electrical characteristics of the device itself can be used as a temperature-sensitive parameter. Conventionally, pulsed DC characteristics are monitored such as the forward voltage-current characteristic of the base-emitter junction of a bipolar transistor. For example, Sage Enterprises Inc. of Mountain View, Calif. manufactures thermal resistance testers which monitor the forward voltage drop of a diode junction, such as the emitter-base junction of a transistor. The selected parameter under steady-state operating conditions is compared with the same parameter shortly after power is applied and before the device has had time to heat significantly. The apparatus advantageously employs the active region being measured as the sensing element. However, in practical use, the apparatus is limited mainly to discrete, single devices in packages that give access to all of the device terminals.
The present invention is directed to an improved apparatus and technique for measurement of the operating temperature of the active region in a semiconductor device where terminals of an active component therein are not readily accessible.